Trace element partitioning between pyrochlore, microlite, fersmite and silicate melts

Stephan Klemme,Jasper Berndt

doi:10.1186/s12932-020-00072-w

Abstract

We present experimentally determined trace element partition coefficients (D) between pyrochlore-group minerals (Ca2(Nb,Ta)2O6(O,F)), Ca fersmite (CaNb2O6), and silicate melts. Our data indicate that pyrochlores and fersmite are able to strongly fractionate trace elements during the evolution of SiO2-undersaturated magmas. Pyrochlore efficiently fractionates Zr and Hf from Nb and Ta, with DZr and DHf below or equal to unity, and DNb and DTa significantly above unity. We find that DTa pyrochlore-group mineral/silicate melt is always higher than DNb, which agrees with the HFSE partitioning of all other Ti–rich minerals such as perovskite, rutile, ilmenite or Fe-Ti spinel. Our experimental partition coefficients also show that, under oxidizing conditions, DTh is higher than corresponding DU and this implies that pyrochlore-group minerals may fractionate U and Th in silicate magmas. The rare earth element (REE) partition coefficients are around unity, only the light REE are compatible in pyrochlore-group minerals, which explains the high rare earth element concentrations in naturally occurring magmatic pyrochlores.

Highlights

To understand the behavior of trace elements in igneous rocks, trace element partition coefficients between minerals and melts are needed
Numerous experimental studies focused on the trace element partitioning between major rock forming minerals and melts [1], but few experimental partition coefficients are available for accessory phases such as rutile, ilmenite, spinel or apatite in basaltic compositions [2,3,4,5,6,7,8,9,10,11,12] and even less data are available for accessory mineral phases such as perovskite or pyrochlore in alkaline rock compositions [13]
Electron microprobe and LA-ICP-MS analyses of pyrochlore and fersmite crystals and quenched melts indicate major- and trace element homogeneity, which is taken as evidence for the attainment of equilibrium between crystals and melts in our runs

Summary

Introduction

To understand the behavior of trace elements in igneous rocks, trace element partition coefficients between minerals and melts are needed. Our experiments presented here were done in simplified chemical compositions, and must be considered as a first step towards a better understanding of trace element partitioning in complex natural systems where pyrochlore-group minerals occur. The choice of this rather simple chemical composition was made after our reconnaissance experiments in more complex systems yielded only small Nb-mineral crystals (< 15 μm), which were impossible to analyze with our LA-ICP-MS set-up. Both starting materials were prepared from analytical grade oxides, hydroxides, and carbonates (Table 1), which were ground in an agate mortar under ethanol. Obtained results match the published range of concentrations given in the GeoReM database [39]

Results and discussion

D Ta-pyr HW3

Conclusions